Xeroradiography, as disclosed in U.S. Pat. No. 2,666,144, is a process wherein an object is internally examined by subjecting the object to penetrating radiation. A uniform electrostatic charge is deposited on the surface of a xerographic plate and a latent electrostatic image is created by projecting the penetrating radiation, such as X-rays or gamma rays, through the object and onto the plate surface. The latent electrostatic image may be made visible by contacting the latent electrostatic image on the plate surface with fine powdered particles (toner) electrically charged opposite to the latent electrostatic image pattern on the plate in order to develop a positive image (in order to develop a negative image, the toner is of the same polarity as the latent electrostatic image pattern). The visible image may be viewed, photographed or transferred to another surface where it may be permanently affixed or otherwise utilized. The entire processing is dry, and no dark room is necessary.
Xeroradiography in recent years has been utilized to examine the extremities, the head, and to detect breast cancer in women. In examination of breasts wherein soft tissue comprises most of the breast area, xeroradiography, or xeromammography as it is generally called, provides greater resolving power than the conventional roentgenographic film and greater image detail is achieved. A wide range of contrast is seen on the xeroradiographic plate as compared to the conventional roentgenographic films so that all the structures of the breast from the skin to the chest wall and ribs may be readily visualized. Besides providing better contrast, xeromammography detects small structures like tumor calcification and magnifies them more than conventional film, is quicker, less expensive, gives greater detail and requires less radiation than prior nonphotoconductive X-ray techniques. The Xerox 125 system marketed by the Xerox Corporation, Stamford, Conn., is a commercially available apparatus for use in xeromammography.
Recent articles by Binnie et al (Application of Xeroradiography in Dentistry, Journal Dent., 3:99-104, 1975) and Gratt et al (Xeroradiography of Dental Structures, I. Preliminary Investigations, Oral Surg., 44:148-157, July 1977 and Xeroradiography of Dental Structures, II Image Analysis, Oral Surg., 44:156-165, 1978) have described the application of the X-ray imaging in dentistry wherein the Xerox 125 system was utilized on phantoms and cadavers. The satisfactory intraoral results provided by this procedure prompted the development of an intraoral radiographic dental system based on xeroradiographic technology which would make the system acceptable to the dental profession.
The original development work on dental xeroradiography was accomplished with powder cloud development on the Xerox 125 System, the first developmental units suitable for intraoral dental xeroradiography employing a miniaturized version of the powder cloud development used in the Xerox 125 System. In addition to being electromechanically very complicated, powder development produced grainy images. Edge deletion control, an important feature in dental radiography where the objects to be x-rayed have high density contrast, could not be established satisfactorily. The lack of adequate deletion control, the presence of undesirable image grain and the complicated design prompted the investigation of other development techniques, and in particular, to liquid development.
Although many liquid toner development systems are commercially available, none of these met the image quality requirements and the high sensitivity characteristics of the toner utilized.